Solar cells using monocrystal, polycrystal or amorphous silicon semiconductors are used these days in electronic devices such as electronic calculators and for residential units. However, since high precision procedures such as plasma CVD and crystal growth processes at high temperature are employed to manufacture solar cells using such silicon semiconductors, a significant amount of energy is required and expensive apparatuses that operate in a vacuum. Thus, the manufacturing cost is high.
Therefore, for solar cells to be manufactured at low cost under normal atmospheric pressure, dye-sensitized solar cells are suggested. In a general cell design, dye-sensitized solar cells have the following: transparent conductive thin film formed on a surface of a transparent substrate; a working electrode which is formed on the transparent conductive thin film and has a porous metal-oxide semiconductor layer made of fine particles with adsorbed dye; an opposing counter electrode made of a catalyst (such as a conductive substrate with platinum or carbon); and electrolyte between the working electrode and the counter electrode. As for the photosensitizing dye, a metal complex containing metal such as Ru, or an organic dye that does not contain metal, is used.
However, a process for dye to be adsorbed on the porous metal-oxide semiconductor layer is conducted by dipping the substrate in a solution with dissolved dye for about half a day in a dark place under atmospheric pressure. Thus, to reduce the time for manufacturing dye-sensitized solar cells and make the process more efficient, a high-speed dye adsorption procedure is required.
As a method for high-speed dye adsorption, the following is proposed, for example: a substrate having a film made of porous metal-oxide semiconductor fine particles is dipped in compressed fluid containing carbon dioxide under temperature and pressure conditions for obtaining supercritical carbon-dioxide fluid with a dissolved Ru complex, a metal complex, as a dye solution. It is reported that a dye-adsorbed film obtained by dipping a substrate for 30 minutes using the above method shows higher photoelectric conversion efficiency than a film obtained by a conventional dipping method (for example, see patent publication (1)).